This invention relates to control systems, and more particularly to a hydraulic control system for effecting the operation of the valves of a subsea christmas tree from a location that is remote therefrom.
It has long been commonplace to effect the production of oil and/or gas from subsea wells. However, for the most part the oil and/or gas producing subsea wells, which have existed heretofore, were drilled at relatively shallow depths. Now, though, in the continuing quest for additional supplies of oil and/or gas, such subsea wells are being drilled with ever increasing frequency at greater and greater depths.
In further reference to the above, once the drilling of such subsea wells at these relatively greater depths has been successfully accomplished, i.e., once the oil or the gas has been found, there still nonetheless remains the task of producing the oil or the gas therefrom. To this end, as is well-known to those who are skilled in this art, the normal practice is to emplace at the subsea well site, a piece of well equipment that most commonly is referred to as a christmas tree. The latter christmas tree is cooperatively associated with the subsea well such that control may be exercised therewith over the production of oil or gas from the well. For this purpose, the christmas tree is suitably provided with a plurality of valve means.
Accordingly, there, therefore, exists a need to establish operational control over this plurality of valve means. However, because of the fact that the christmas tree and thus the valve means thereof are positioned at the subsea well site, which is located at a considerable water depth, control is generally exercised over the valve means from a remote location. In turn, this necessitates the use of some form of control means that is operative notwithstanding the fact that it must be capable of operating over an extended distance, i.e., between the subsea well site whereat the christmas tree is located and the remote location from whence it is desired that control over the valve means of the christmas tree be effectuated.
Apart from simply being capable of accomplishing therewith the requisite operation of the valve means of the subsea christmas tree, any control system that may be selected for use in this regard should also be advantageously characterized in certain other respects. For instance, additionally such a control system should be characterized by the fact that it is capable of performing in a substantially trouble free manner even though operated under conditions which demand extensive usage therefrom. That such a characteristic is desirable will be readily apparent to all when one considers the location of the valve means over which the control system is intended to effect control, and when one envisages the difficulties of having to make repairs on the control system at the site of the valve means should a need therefor arise. Another characteristic, which it is desirable that such a control system possess, relates to the cost of providing the system. Namely, because of the fact that the control system is required to function over an extended distance as well as in a considerable depth of water, it is desirable that the means by which the control system accomplishes the control function over the valve means of the subsea christmas tree be advantageously characterized as regards both its relative simplicity as well as its relative low-cost.
Notwithstanding the fact that the control system, which is selected for use for the purpose found described above, may be advantageously characterized in that it is capable of providing relatively trouble free operation, there still exists a need for effecting normal maintenance thereon. Moreover, the ease with which such maintenance can be performed, both in terms of the time and effort that must be expended in the accomplishment thereof, is an important consideration for the user of such equipment. To this end, when wells were being drilled offshore at relatively shallow depths, it was found to be both feasible and desirable to utilize divers to accomplish the required maintenance and/or repairs on the equipment located at the subsea well site including such items of equipment as the christmas tree itself, the valve means associated therewith, etc., as well as the control system extending between the subsea christmas tree and the surface control center from whence control is exercised over the valve means of the christmas tree. However, as production wells are being drilled with increasing frequency in deeper and deeper depths of water, it is becoming no longer feasible to make use of divers for purposes of accomplishing the maintenance and/or repair functions that are referred to above. Rather, there is being generated a need for the utilization of control systems that possess ever increasing reliability, as well as control systems that are ever less costly to provide.
There have been attempts made with varying degrees of success by the prior art relative to addressing the need for new and improved forms of control over the production of oil or gas from subsea well sites. That is, in accordance with the teachings of the prior art, there are known to have been provided heretofore various forms of subsea production control systems operative for purposes of exercising from remote locations control over subsea christmas trees. The latter christmas tree, as has been previously mentioned herein, are designed to be emplaced on oil or gas wells in order to control with the trees the flow of product from the wells. Typically, the christmas trees include a series of valve means having hydraulic operators by means of which the opening and the closing of the valve means is accomplished. To this end, upon pressurization with a hydraulic fluid, the hydraulic operators of the valve means cause the latter to occupy either an open or closed position depending upon what the orientation of the valve means is at the time of the pressurization. Thereafter, upon depressurization of the hydraulic operators of the valve means, the latter are caused to occupy the other position thereof, i.e., are made to assume their original positions. Namely, if prior to pressurization the valve means occupied a closed position, upon pressurization of the hydraulic operators the valve means are caused to move from the closed position to an open position, and upon depressurization of the hydraulic operators the valve means are made to reassume their closed position, i.e., are caused to move from an open to a closed position. On the other hand, if the valve means are in an open position prior to the pressurization of the hydraulic operators thereof, the opposite will occur. Namely, upon pressurization the valve means will move to the closed position, and upon depressurization the valve means will reassume the open position.
By way of a generalization, it is possible to categorize prior art forms of subsea production control systems according to the manner in which the actuation of the hydraulic operators of the valve means of the subsea christmas tree is accomplished. Further to this point, it can be said that there exist subsea production control systems which are classifiable into the following categories: direct hydraulic systems, sequenced hydraulic systems, electro hydraulic systems and multiplexed electro hydraulic systems. For purposes of the discussion that follows hereinafter, though , attention is focused in particular upon that category of subsea production control system that has been referred to above as a sequenced hydraulic system.
Basically, a sequenced hydraulic system is a system wherein there are provided by means of a pilot line specified hydraulic pressure levels corresponding to certain operation modes for a christmas tree, and these specified pressure levels are employed for purposes of effecting control over the operation of the valve means that are cooperatively associated with the christmas tree. As regards prior art forms of sequenced hydraulic systems, they have all suffered in one respect or another from some type of readily identifiable shortcoming or deficiency. In particular, prior art forms of sequenced hydraulic systems suffer insofar as concerns the capability thereof to maintain reliability through simplicity, and concomitantly, therefore, enable economies to be realized in terms of the cost of providing the system as well as the cost to the user of employing the system.
Essentially, it can be said that all prior art types of sequenced hydraulic systems embody some form of valving means that is designed to be connected in operative relation to the valve means of the subsea christmas tree. More specifically, all such prior art types of sequenced hydraulic systems embody a form of construction which is dependent for its operation upon the fact that a pressure bias is applied to the aforesaid valving means which the subject sequenced hydraulic system embodies. Unfortunately, however, in order to utilize a mode of operation which makes use of a pressure bias it necessitates the running of an additional hydraulic line between the subsea christmas tree and the surface control center from whence control is exercised over the valve means of the subsea christmas tree. The requirement of providing such an additional hydraulic line serves to disadvantageously characterize from a cost standpoint those prior art sequenced hydraulic systems that make use of a pressure bias mode of operation. For an example of such a prior art type of sequenced hydraulic system that utilizes a pressure bias mode of operation, reference may be had to the product literature of the Hydril Company.
Another prior art type of sequenced hydraulic system comprises the subject matter of U.S. Pat. No. 3,993,100. In accordance with the teachings of this patent, a sequenced hydraulic system is provided that does not require a separate hydraulic line to effect the biasing of the valving means which the system embodies. Rather, the sequenced hydraulic system as taught in U.S. Pat. No. 3,993,100 utilizes a subsea mounted regulator means for purposes of effecting the pressure bias that is applied to the valving means of the system. Although such a system avoids the costs associated with providing an hydraulic line between the subsea christmas tree and the surface control center, this system likewise is disadvantageously characterized. More specifically, the shortcoming of the sequenced hydraulic system that forms the subject matter of U.S. Pat. No. 3,993,100 resides in the use of the subsea mounted regulator means. Such regulator means have proven in the past to be unreliable. Moreover, they have exhibited a tendency to be subject to drift and pressure variations. Accordingly, because of the uncertainty shown by such subsea mounted regulator means, the pressure biasing forces to which the valving means of the system is subjected can change thereby resulting in the establishment of erroneous biasing forces which concomitantly can lead to the incorrect exercise by the system of control over the operation of the subsea christmas trees, i.e., can result in faulty control being exercised over the flow of product from the subsea well with which the christmas tree being controlled by the system of U.S. Pat. No. 3,993,100 is cooperatively associated.
Yet another illustration of a prior art type of sequenced hydraulic system can be found described in the product literature of Cameron Iron Works. In this system, reliance is had solely on the use of the supply pressure of the hydraulic fluid to effect the biasing of the valving means of the system. Although this method of accomplishing the pressure biasing of the valving means of the system eliminates the need both of running an additional hydraulic line from the surface control center to the subsea christmas tree and of making use of a subsea mounted regulator means, the system is highly dependent upon the existence of a very stable supply pressure. This dependency serves to disadvantageously characterize such a system inasmuch as a flow demand at the subsea location in combination with the presence of normal restrictions in the hydraulic supply line can cause pressure drops to occur in the latter supply line, which as noted establishes the biasing pressure to which the valving means of the system is subjected. In turn these pressure drops can cause the establishment of uncontrollable bias force variations on the valving means of the system and thus produce erratic performance, i.e., faulty control over the flow of product from the subsea well.
Yet still another example of a prior art type of hydraulically actuated subsea production control system is the indexing type of system provided by the FMC Corporation. Although this latter type of system is characterized by the fact that it overcomes some of the shortcomings from which prior art forms of sequenced hydraulic systems suffer, it does not itself make use of valving means that functions in a sequential fashion and which is responsive to predetermined pressure levels that are established in a pilot line. Thus, this indexing type of system fails to address the need which has been shown to exist for a sequenced hydraulic system that possesses the capability of being able to maintain therewith reliability through simplicity and thus further of being able to reduce therewith the cost of providing the system as well as the operating costs to the users of employing the system.
It is therefore, an object of the present invention to provide a new and improved form of a subsea production control system, which is operative for purposes of effecting control over the operation of a subsea christmas tree from a surface location remote therefrom.
It is another object of the present invention to provide such a subsea production control system which is operative in the manner of a sequenced hydraulic system; namely, the system includes valving means that is operative in a sequential fashion in response to the existence of certain predetermined pressure levels in a pilot line.
It is still another object of the present invention to provide such a subsea production control system which is advantageously characterized in that it minimizes both the number of control lines that are required to be run between the subsea christmas tree and the surface control center from whence control over the subsea christmas tree is effected, and the number of subsea control valves required for operation of the system.
Another object of the present invention is to provide such a subsea production control system that possesses the flexibility of enabling a variety of different operating programs for subsea christmas trees to be executed therewith.
A further object of the present invention is to provide such a subsea production control system that utilizes a new and improved form of sequence valving means, the latter being characterized both in that it has a small dead band and in that it permits the elimination of the use of shuttle valves interposed between the sequence valving means and the valve means of the subsea christmas tree.
A still further object of the present invention is to provide such a subsea production control system embodying such a new and improved sequence valving means wherein the latter sequence valving means is designed to be located in or on the tree cap of the subsea christmas tree.
Yet another object of the present invention is to provide such a subsea production control system embodying such a new and improved sequence valving means wherein the latter in turn embodies a new and improved constant force spring means which is operative for purposes of effecting a biasing of the gate means of the sequence valving means such that the latter possesses a mode of operation wherein the biasing thereof is accomplished by both constant force mechanical spring means and hydraulic pressure, and wherein assistance is provided in the form of hydraulic pressure in the shifting of the internal components of the sequence valving means to cause the latter to occupy either an open or closed operating position.
Yet still another object of the present invention is to provide such a subsea production control system embodying such a new and improved sequence valving means wherein the latter in turn embodies a new and improved low friction duplex shear seal means that is operative to prevent interflow within the sequence valving means.
Yet a further object of the present invention is to provide such a subsea production control system which allows for the use of a multi-piston control valve means located in the fluid lines that function to fluidically interconnect the sequence valving means with the valve means of the subsea christmas tree.